“One of the good things we’re seeing this cycle is it’s not even an afterthought,” Mr. Skatell said. “They’re coming and saying, ‘Let’s do digital,’ and then if there are constraints on TV, they’re coming back again and saying, ‘Let’s buy more digital.’ ”

In July, Robert Willington, the president of Swiftkurrent, a Republican digital marketing company that is working on Scott Brown’s Senate campaign in New Hampshire, purchased “a lot” of the available YouTube inventory in the state for future ads.

And in August, IMGE, a digital advocacy agency handling online strategy for Karl Rove’s Crossroads GPS, snapped up all available YouTube inventory in Alaska for the final weeks of the Senate race there.

In June, Tim Lim, the president of Precision Network, a digital media buyer for Democrats, began purchasing ad space from premium online vendors — Google, Yahoo, Pandora — in Senate battleground states like Colorado and Iowa.

Mr. Lim of Precision Network, who is working with the Democratic House candidate Staci Appel in Iowa, began talking to The Des Moines Register back in August about a home-page takeover, which he reserved for the first two days of early voting in Iowa in September.

“It just shows digital advertising has gotten a lot more sophisticated, and people are planning a lot more in advance,” Mr. Lim said. “If campaigns want that premium space, it requires them to act a lot quicker than they’re used to.”

“Smart campaigns book early, the same way that smart brands book television early,” said Andrew Bleeker, the president of Bully Pulpit Interactive, a Democratic digital marketing company. “We reserve most of the inventory for our clients in the spring to make sure something like this isn’t a factor.”